Transcript Abstract: Human life quality and preservation of ecosystems

Prototype for Water Conductivity Measurements
José Gouveia1, Pedro Ramos1, Helena Ramos1, Artur Ribeiro1,J.M. Dias Pereira1,2
1Instituto de Telecomunicações, Av. Rovisco Pais, 1049-001, Lisboa, Portugal, Emails:
[email protected], [email protected], [email protected], [email protected]
2Escola Superior de Tecnologia de Setúbal, Instituto Politécnico de Setúbal, Rua do Vale de
Chaves, Estefanilha, 2914-508 Setúbal, Portugal, Email: [email protected]
Abstract: Human life quality and preservation of ecosystems are two major topics that depend on environmental quality.
In this poster particular attention is dedicated to water conductivity assessment in estuarine zones. Estuarine zones are
generally surrounded by people concentration, that use those places as residential zones, and also by industrial plants that
take advantage of water availability to generally cool, and sometimes warm (e. g., liquid natural gas plants), their
industrial equipments. At the same time boats, particularly those that transport hydrocarbons, represent also a danger that
can affect water quality. This poster includes the design and implementation of a low cost conductivity sensor, its
metrological characterization and a detailed description of the calibration techniques used to compensate errors caused by
temperature variation.
Main characteristics and advantages of the implemented prototype
• Low cost, insensitivity to contact resistance and fringing effects provided by four-terminal resistive element measuring
principle and a wide measurement range that can be extended by controlling the cell’s excitation voltage level.
• Easy implementation of a fouling detector triggered by cell excitation voltages. This capability is of paramount importance
due to the harsh environmental conditions to which these sensors are generally exposed.
• Linear behavior that enables an accurate calibration using a minimal number of standard solutions.
System description
Conductivity Sensor
Conditioning circuit
90 mm
45mm
35mm
20 mm
Measurement method and data processing
For the conductivity sensor conditioning circuit, an
ADSP-BF533 was used with an AD1836 codec, for data
acquisition and generation. The ADC data records are
transmitted to the DSP by a SPI connection. One of the
DAC’s in the AD1836 is used to generate the sine
stimulus for the impedance measurement circuit. To
measure the temperature, a TMP36 temperature sensor
is used together with a AD974 16-bit ADC. The
temperature measurements are taken each 100 ms.
An improved sine-fitting algorithm is used to extract
voltage
and
current
amplitudes,
phases,
DC
components and frequency.
2 mm
35 mm
45 mm
20 mm
16 mm
20 mm
HI CUR
HI POT
LO CUR
LO POT
(a)
(b)
(a) longitudinal section view
(b) transversal section view
Results
Discussion
20
30
10 3
2200
25
20
15
Measurement
Interpolation
10
Impedance Amplitude (Ω)
18
Admitance Amplitude (S)
Real Temperature (ºC)
Considering the injected
current between sensing
terminals the cell geometry
factor, based on the FEA
simulation program is given
by:
-1
2400
35
16
14
12
10
8
Measurement
6
Interpolation
4
2
5
0,0
0
5
10
15
20
25
30
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
Conductivity (S/m)
T=15 ºC
1600
1,0
ksm=Rsm×σ=52.466 m
T=20 ºC
1400
The cell geometry factor
based
on
admittance
measurements:
T=25 ºC
T=30 ºC
1000
0
Temperature Measured by TMP36 (ºC)
Real temperature (measured by a
calibrated system) as a function of
the temperature measured by the
temperature sensor TMP36
T=10 ºC
1800
1200
0
0
T=5 ºC
2000
2000
4000
6000
8000
10000
12000
frequency (Hz)
Conductivity sensor admittance
amplitude as a function of the liquid
conductivity for f = 1000 Hz and
T=25 ºC.
Conductivity sensor impedance
amplitude as a function of the liquid
temperature and sine frequency.
Y =-5.97×10-4+1.839×10-2×σ
gives
an
experimental
value of the cell geometry
factor of 54.37 m-1.
The worst relative error of
the estimated conductivity
is lower than 1.86 %.
Conclusions - The proposed prototype is an attractive solution for water quality measurements systems in estuarine zones. Main
characteristics of the proposed prototype include automatic temperature compensation of conductivity measurements, low
sensitivity to disturbances caused by electrolytic polarization, double layer and fringe effects.
Experimental impedance results of the conductivity cell, and its variation with frequency and temperature, confirm theoretical
expectations. A good agreement between simulation and experimental results validates the theoretical assumptions of the
prototype’s model.
Work sponsored by project DICSAP/WCA – IT/LA/298/2005 financed by Instituto de Telecomunicações